1. Field of the Invention
This invention relates to a fault locator for electric cables, such as, telephone, power, or video cables, and more particularly to a system capable of locating high resistance shorts without fusing or excessively damaging the conductors and/or the insulation surrounding the conductors.
2. Description of the Prior Art
One technique for locating troubles or faults in a telephone cable is to apply to the cable pair between which there is a high resistance short a high voltage which welds the conductors of the pair to one another at the location of the fault to produce a hard short. The hard short can be located readily with bridge circuits and tone devices but the welding procedure, because it generates substantial heat, is likely to adversely affect conductor pairs in proximity to the defective pair.
Distance measurement techniques have also been employed, resistance and capacitance bridge circuits and time domain reflectometers being exemplary of such distance measurement circuits. These circuits are subject to inaccuracies due to variations in wire resistivity and capacitance plus unknown factors such as the degree of spiraling of the conductors in the cable and trench and the existence of buried loops. Such inaccuracies can be large enough to make the resultant measurements unusable in longer runs of cable. The resistance bridge technique is further complicated by the necessity of making connections at a far end of the cable and of having a good conductor pair in parallel with the pair under test. The time domain reflectometer is subject to similar distance inaccuracies and has limited usefulness because pulses produced thereby will be reflected only by a fault that has a resistance sufficiently low that when shunted by the characteristic impedance of the conductors will produce an impedance discontinuity which will reflect the pulse. Thus, unless there is a very low resistance short or a hard short, a time domain reflectometer does not locate a fault. Failure to accurately locate a fault, particularly in the case of buried cable, is inconvenient because excessive digging must be performed to locate and repair the fault. For this reason it is desirable to use a tone to pinpoint the location of the fault after using a bridge or time domain reflectometer to find its general locale, because the tone is not subject to the above mentioned distance inaccuracies. The tone, however, will travel past a fault unless it is a low resistance fault. Thus, the desirability of a method and apparatus for temporarily reducing the resistance of the fault without damaging the cable can be appreciated.
Another distance measuring system is disclosed in U.S. Pat. No. 4,023,154 (340/256). As is the case with a time domain reflectometer, the accuracy of the patented system functions well only with respect to certain types of cable faults, e.g., a hard short.
There is an impulsed arc technique that is used in power cables for locating ground faults. This technique charges a capacitor which is discharged between ground and the line containing the fault. This arcs over the fault and the current flow through the line and the arc causes a rapidly rising magnetic field which produces a "thump" that can be detected by a receiver coil, or by a sonic pickup. The technique has little use in conductor-to-conductor faults, however, because the magnetic fields produced by current flow through the two conductors cancel one another so that almost no signal can be received.
This technique has a further disadvantage in that the capacitor must be charged to a high voltage to start the arc, but one initiated, the arc has a very low voltage so most of the energy stored in the capacitor is wasted or dissipated in circuit resistances during discharge. Thus the thumper consumes a large amount of power and is heavy and inefficient. Further, it dissipates excessive energy in the arc and generates a large transient which precludes its use on telephone cable.
Two printed publications that summarize various fault locating procedures are:
1. Evaluation of Underground Fault Location Techniques, EPRI TD-153, April 1976, Electric Power Research Institute, and;
2. Underground Cable Fault Location: A Handbook to TD-153, EPRI EL-363, January 1977, Electric Power Research Institute.